WASHINGTON President Donald Trump's explosive allegation that his predecessor wiretapped his Manhattan skyscraper wilted further yesterday as two high-ranking senators and the top Republican in Congress said they saw no evidence to back the claim.

The Trump's administration is facing calls to either shore up, or drop, the unsubstantiated claim that former President Barack Obama ordered the phones tapped at Trump Tower during the election campaign.

Upping the pressure, a statement from both the chairman of the Senate Intelligence Committee, Republican Richard Burr, and the committee's Democratic vice-chair Mark Warner, said they had seen no information to support Mr Trump's allegation, made in tweets on March 4.

"Based on the information available to us, we see no indications that Trump Tower was the subject of surveillance by any element of the United States government either before or after Election Day 2016," they said.

The clear verdict from the committee leaders comes a day after two heads of the House of Representatives Intelligence Committee said they had received no information to back the Trump tweets.

Their rebuttal of Mr Trump's claim was further echoed by House Speaker Paul Ryan, the top Republican on Capitol Hill, who told CNN yesterday that "we have not seen any evidence that there was a wiretap" - although he also argued the false claims would not damage Mr Trump's credibility.